Traction control method for slipping wheels of at least one driving axle

ABSTRACT

The present invention relates to a method for traction control of slipping wheels of at least one driving axle, which are driven by way of an open differential gear and can be interlocked by braking intervention to a defined, predeterminable degree for effecting a differential lock function. The invention discloses that the wheel slip of the wheels is detected, that the detected wheel slip variation is compared with predetermined wheel slip patterns for recognizing and monitoring a current driving condition, and that a braking intervention effecting the differential lock function is performed when the sensed wheel slip variation coincides with at least one predetermined wheel slip pattern. The invention allows an accelerated adjustment and control of a differential lock effect depending on the respective driving condition.

TECHNICAL FIELD

The present invention relates to a traction control method for slippingwheels of at least one driving axle, which are driven by way of an opendifferential gear and can be interlocked by means of brakingintervention to a defined, predeterminable degree for effecting adifferential lock function.

BACKGROUND OF THE INVENTION

To avoid the spinning of driving wheels, it is known in traction controlto arrange for an intervention into the wheel brake and, as the case maybe, additional intervention into engine management. The wheel brakeintervention is executed on wheels subjected to traction slip andpermits a differential lock effect. This procedure is based on theconsideration of decelerating the wheel subjected to traction slip byway of a defined brake torque to achieve a rotational speed with afavorable predetermined traction slip so that using an open differentialgear allows the brake torque generated on one side to be available as anadditional drive torque at the other, opposite wheel. Details can betaken from the technical book ‘Fahrsicherheitssysteme’ (Driving SafetySystems), second edition Vieweg 1998, page 75 sqq.

DE 34 21 776 C2 discloses a vehicle with all-wheel drive, wherein thewheels of an axle are connected to the drive shaft and the drive shaftsare connected to the driving motor by way of differential gears. Anelectronic differential lock effect is achieved by introducing a brakingpressure on each individual wheel subjected to traction slip. Morespecifically, braking irrespective of the driver will take place with acorresponding brake torque when traction slip is detected, with a viewto reducing the effective drive torque at the spinning wheel. The dosingand modulation of the brake torque is based on slip control on eachindividual wheel.

A traction control method for use with open differentials is known fromthe technical journal ATZ 102(2000) No. 9, pages 764–773, wherein, likein anti-lock control, it is detected by evaluation of wheel speedsignals whether the wheels are still in a predetermined slip range.During traction control, braking intervention is effected on wheels thatexceed the slip threshold. An active braking pressure build-up canprevent a high degree of traction slip on respectively diagonallyopposite driving wheels.

A method of detecting so-called axle twist conditions with diagonallyopposite slipping driving wheels is disclosed in DE 199 53 773 A1. Nohint can be taken from this publication to carry out a detection ofdriving conditions beyond axle twist conditions.

BRIEF SUMMARY OF THE INVENTION

An object of the invention involves improving the prior art controlmethods with an open differential in such a fashion that an acceleratedadjustment and control of the differential lock effect is renderedpossible in dependence on the respective driving condition including theprevailing states of driving (coefficient-of-friction situation).

This object is achieved by the features of the described tractioncontrol method in that the wheel slip of the wheels is sensed, that asensed wheel slip variation is compared with predetermined wheel slippatterns for detecting and monitoring a current driving condition, andthat a braking intervention effecting the differential lock function isperformed when the sensed wheel slip variation is coincident with atleast one predetermined wheel slip pattern.

The invention is based on a method for traction control of slippingwheels of at least one driving axle which are driven by way of an opendifferential gear and can be interlocked by means of brakingintervention to a defined, predeterminable degree for effecting adifferential lock function. The wheel slip of the wheels is sensed, asensed wheel slip variation is compared with predetermined wheel slippatterns for detecting and monitoring a current driving condition, and abraking intervention effecting the differential lock function isperformed when the sensed wheel slip variation is coincident with atleast one predetermined wheel slip pattern. The invention permitsperforming driving-condition-responsive braking interventions with adifferential lock function by using reasonable effort. In drivingsituations where opposite driving wheels are slipping, the inventionallows a quick and effective traction control.

For the quick control of the locking torque, an adaptive pressurereduction gradient or an adaptive pressure increase gradient iscalculated on each individual wheel in dependence on the detected wheelslip pattern, and the pressure reduction or the pressure increase isrealized by adapting pulse-pause valve actuation sequences.

Advantageously, a pause between two pressure reduction pulses or betweentwo pressure increase pulses is found out on the basis of data relatingto a vehicle reference speed, the wheel slip and, as the case may be, aconstant base quantity. In addition, the slip behavior of the wheels canbe monitored additively, and a progression or degression of a counterwill occur in dependence on the slip behavior. A difference in torquesbetween a current axle torque and a filtered axle torque and datarelating to the rotational behavior of a driving engine can beconsidered to prevent stalling of the engine.

It has proven advantageous that the used electronic controller,preferably software-based, is based on a controller model with linearcomponents (P-components), integrating components (I-components) and, asthe case may be, differentiating components (D-components, delayelements), and the above-mentioned influences can be superimposed asdisturbance variables.

A loss in traction is counteracted in an especially early fashion whenthe amplification factors (k₁₋₅, k_(p), k_(d)) are determined each independence on the detected driving condition of the vehicle, and whenthe control threshold at the slipping wheel (pair) is lowered afterdetection of a driving condition with traction slip.

To render the differential lock function plausible, a test will beperformed based on the vehicle reference speed and based on a corneringdetection. If a differential lock does not appear plausible, it will bedeactivated. Further, deactivation is performed when a driving conditionis detected where all wheels are on a low coefficient of friction orwhen cornering prevails. Cornering detection can be carried out in asimple fashion by monitoring the steering angle.

According to a favorable aspect of the invention, a traction controlstrategy based on wheel-individual slip values is possible and providedin addition to the traction control strategy based on identified drivingconditions, and the control is performed on the basis ofwheel-individual slip values when the differential lock function wasdeactivated, in particular when a driving condition was detected whereall wheels adopt a low coefficient of friction.

To determine the vehicle reference speed free from traction controlinfluences, the relating rotational data of traction-controlled wheelsis not taken into account. When a traction control intervention prevailsat all vehicle wheels, the driving wheel with the lowest locking torque(brake torque) due to traction control is taken into account fordetermining the vehicle reference speed.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing, the only FIG. 1 is a schematic diagram forillustrating a difference between the current and the filtered axletorque.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to a method for traction control of slippingwheels of at least one driving axle of a motor vehicle, which wheels aredriven by way of an open differential gear and can be interlocked bymeans of braking intervention to a defined, predeterminable degree toeffect a differential lock function. Principally, an all-wheel drivenvehicle is equipped with a transverse differential gear between thewheels of a driving axle. A longitudinal differential gear is interposedin a connection between two different driving axles. Open differentialgears act as a torque balance, and the respective output speed isconformed to the conditions. Different wheel speeds of the inside wheelin a turn and the outside wheel in a turn of one axle are realizedduring cornering, for example. It is self-explanatory that enginecontrol intervention is possible in addition to braking intervention.Engine control interventions are considered as principally known and,therefore, do not need any further explanation.

Different driving situations can be encountered with all-wheel drivenvehicles—mainly when starting to drive or when driving on roadways withdifferent coefficients of friction, on rough road sections or inoff-road operations—which can be recognized by way of the wheel slipvariation (plotted as a function of time). Further, special cases arefeasible such as conditions with abrupt μ changes, with one or bothwheels of an axle on a low coefficient of friction (LM) and the wheelsof the other axle on a high coefficient of friction (HM). Besides,μ-split situations are possible, where the wheels of one vehicle sideare confronted with a high coefficient of friction and the wheels of theother vehicle side are confronted with a low coefficient of friction. Inaddition, mixed and special forms are feasible if, for example, theintroduced engine torque cannot be applied e.g. due to lifted wheels orwheels greatly relieved from vertical forces, for example in axle twistsor on loose underground at individual or all driven spinning wheels sothat no propulsion is produced. It is therefore known to build up alocking torque (brake torque) on slipping wheels, which are unable totransmit a driving torque, by means of a braking pressure buildupirrespective of the driver so that the driving torque can be transmittedby driving wheels, which are not slipping at all or are slipping to alower degree.

When the term ‘slip’ has been referred to hereinabove, basically thebrake slip measured by way of wheel speed sensors was meant which canprincipally adopt values between 0% (freely rolling) and 100% (locked).According to the above terminology, brake slip—in contrast to tractionslip—has a negative sign. The measured slip can be positive in thecourse of a traction control intervention when temporarily traction slipoccurs again, e.g. after a first control cycle.

To adapt traction control to the mentioned driving conditions and, inparticular, adjust an adaptive differential lock rate with a definedlocking torque, ‘adaptive’ meaning adapted to conditions, the slip ofthe wheels is sensed, and the sensed wheel slip variation is comparedwith predetermined wheel slip patterns for detecting and monitoring acurrent driving condition. Only when the sensed wheel slip variationcoincides with at least one predetermined wheel slip pattern willbraking intervention with a defined differential lock function follow.More specifically, a defined locking rate is adjusted adaptively withouta wheel-individual control according to a nominal slip being performed.When coincidence lacks, control according to a nominal slip will takeplace. Even if a threshold value in the form of an off-road counter isnot exceeded, control according to the nominal slip will take place. Themethod is explained in more detail hereinbelow.

Wheel slip variations shall be classified as follows:

no wheel_or_three_wheels_(—) no obvious slip pattern or three slippingwheels under traction control one_wheel_slipping one wheel undertraction control all_wheels_slipping all wheels under traction control,all wheels on gravel, mud or low coefficient of friction - change toslip-based control ! first_diagonal_slipping diagonally opposite leftfront and (wheels 1 + 3) right rear wheels under traction controlsecond_diagonal_slipping diagonally opposite right front and (wheels 2 +4) left rear wheels under traction control front axle_slipping abruptμ-change situation, only the wheels of the front axle under tractioncontrol rear axle_slipping abrupt μ-change situation, only the wheels ofthe rear axle under traction control left_wheels_slipping μ-splitsituation, wheels of the left side on low-μ right_wheels_slippingμ-split situation, wheels of the right side on low-μ

The following operations are performed on each individual wheel.Individual counters summing counted values (stability counters,LM-counters (for a low coefficient-of-friction situation)) areassociated with each vehicle wheel. Each of the above-mentioned drivingconditions exhibits defined, predetermined counted values so that thedriving conditions can be recognized from the counts of the counters.When a defined count (control threshold) within the respectivemonitoring period is exceeded, the differential lock function isperformed on the basis of individually (adaptively) determinedamplification factors. Otherwise, the differential lock function iscanceled. The differential lock function is also canceled in the eventof unplausibility. A counting method for diagonally offset slippingwheels can be taken from DE 199 53 773 A1. The respective disclosure isreferred to in full extent for detecting different driving conditions.

The state of plausibility is achieved as follows. A poll is made whethercornering exists or is desired by the vehicle operator. This can e.g. bedone in that cornering is measured by way of the prevailing lateralacceleration directly by means of a lateral acceleration sensor. Thesteering angle can be measured according to another variant, and afunctional interrelation between the existing steering angle and thelateral acceleration is used for cornering detection. It is finallypossible to calculate the lateral acceleration from the relation

$a_{y} = {\frac{{SWA}*v_{{Re}\; f}^{2}}{l}\mspace{14mu}{where}}$

-   SWA: steering angle-   ν_(Ref): vehicle reference speed-   l: center distance.

When cornering is detected, the locking rate control is canceled.Further, the locking rate control is deactivated when it is detectedthat all driving wheels exhibit a low coefficient of friction. Thelocking rate control is also deactivated when it is detected e.g. on thebasis of a brake light switch signal that brake application by thedriver prevails. A control on the basis of predetermined slip valuestakes place for these cases.

As stated hereinabove, an adaptive (differential) locking rate controlwith a locking torque modulation (brake torque modulation) on the basisof individually determined controller amplification factors (k values)is executed in dependence on the detected driving condition. The drivingconditions or wheel slip patterns are determined by logical evaluationof the control phases or the wheel slip variation of the individualwheels because it is evaluated how many wheels have exceeded a controlthreshold (slip threshold) within the predetermined monitoring period,and which is the arrangement of the wheels relative one another. Thisdriving condition monitoring is effected continuously in order to reactto condition variations.

The adaptive control of the locking torque (wheel brake torque) iscarried out for each individual wheel by way of a pressure increasegradient or pressure reduction gradient in dependence on the wheel slippattern e.g. determined according to the above table. As the requirementmay be, pressure increase or pressure reduction is realized by theadaptation of pulse-pause valve actuation sequences. Practically, it isa matter of increasing or reducing the braking pressure adjusted on aslipping wheel, delayed or boosted in a defined fashion.

A period of a pause between two successive pulses relating to thepressure reduction is described byΔT=T _(B) −k ₁ *ν _(ref) +k ₂ *λ _(fil) −k ₃*Stab_(counter) −k ₄*ΔActTq−k ₅ *[k _(p)*(N _(nom) −N)+k _(d) *ΔN]where

-   T_(B): base quantity-   k₁, k₂, k₃, k₄, k₅, k_(p), k_(d) amplification factors-   ν_(ref) vehicle reference speed

${\lambda_{fil}\text{:}\mspace{25mu}{brake}\mspace{14mu}{slip}\mspace{14mu}({filtered})} = {\frac{v_{ref} - v_{wheel}}{v_{ref}}*100\mspace{11mu}\%}$

-   ν_(wheel): wheel speed-   Stab_(counter): counter for monitoring the slip behavior (stability    counter)-   ΔActTq: ActTq_(fil)−ActTq-   ActTq_(fil): axle moment (filtered)-   ActTq: axle moment (current)-   N_(nom): nominal engine speed-   N: actual engine speed-   ΔN: engine speed variation.

When necessary, the intensity of a pulse, its amplitude, in particular apressure increase gradient, can be adapted in dependence on the wheelbehavior.

The controller with disturbance variable compensation comprises linear(P) components, integral (I) components and damped (D) components.

As can be seen in the above equation, a reduction phase is initiallydependent on the constant base quantity T_(B), and on the filtered wheelslip λ_(fil) and the vehicle reference speed ν_(ref). A reduction of thepause time ΔT is provided (according to tendency) when the vehiclereference speed ν_(ref) rises, or the slip λ_(fil) rises. Besides, thewheel slip behavior can be taken into account by way of the counterStab_(counter). The counter is raised when the measured slip λ_(fil)lies within predetermined slip ranges, which define control thresholdsabove and below a vehicle reference. If this is not the case—when theslip values lie outside the slip ranges—the counter is reduced. Thispermits monitoring the inherent stability of allegedly tractionlesswheels. It must be added that the control thresholds are lowered duringa traction control operation for the wheels concerned. Morespecifically, the mentioned slip ranges are configured to be narrower inorder to early counteract a loss in traction. When the controlthresholds are exceeded after the decrease, there will be awheel-individual pressure increase with a pressure increase gradientthat is determined adaptively (on the basis of the currently prevailingconditions) in order to thereby redevelop the necessary locking torqueand maintain the slip of tractionless wheels in the defined range aroundthe driving speed.

The wheel torque difference ΔActTq is provided as another superimposeddisturbance variable. This variable considers a dynamic balance and achange in the balance between the currently output engine torque and thegenerated locking torque (brake torque) for the case that the currentaxle torque ActTq is lower than the filtered axle torque ActTq_(fil).This variable renders it possible to influence the pause time for apressure reduction as a function of the dynamics of the driving torquedemanded by the driver. When the vehicle operator e.g. spontaneouslyreduces the demanded driving torque by releasing the accelerator, as issketched in FIG. 1, ΔActTq will prevail at a time t₁ contributing to anadditional reduction of the pause time ΔT. In contrast thereto, thepause time ΔT is influenced only slightly or not at all by graduallyreleasing the accelerator, because at a hypothetical time t₂ afterreleasing the accelerator at t₀, the value of ActTq_(fil) has alreadysignificantly approached the value of ActTq due to a largely linearlydeclining characteristics. Consequently, the difference produced leadsto a reduced influence on the pause time ΔT.

It is particularly significant to monitor the engine rotational speed Nwhile considering the rotational speed change in order to avoid stallingof the driving motor on account of a traction control intervention.Taking into account this control reserve is done on the basis of a PDcontrol approach by way of corresponding amplification factors with aterm, which is predominantly directed to weighting the differencebetween the nominal engine speed N_(nom) and the actual engine speed Nand to the detected change in engine speed ΔN. As can be seen, a pausetime ΔT for a pressure reduction is reduced when the actual engine speedN drops below the nominal engine speed N_(nom). If the engine shows astalling tendency nevertheless, the preset nominal slip is increasedlinearly and the pressure increase gradient reduced, as the case may be.In drastic cases it is possible to change into a traction controlstrategy on the basis of a nominal slip specification. In terms of astalling protection, reference is made in this respect to DE 100 27 628A1 which is included in its full scope.

As can be seen from the above equation, the influencing variables aresubtracted from the base quantity T_(B) in order to arrive at the pausetime ΔT. Also the quantity included by way of slip λ_(fil) is subtractedfrom the base quantity T_(B) due to its negative sign (brake slipappears with a negative sign during traction control).

In a preferred aspect of the invention, not only a traction controlstrategy based on detected driving conditions (adaptive locking ratecontrol) but another traction control strategy based on wheel-individualslip values is provided. When the conditions for the driving conditionstrategy (adaptive locking rate control) do not prevail, said isdeactivated, and switch-over is made to the traction control strategybased on the slip values λ_(fil). It is self-explanatory that startingfrom the traction control strategy based on the slip values λ_(fil) itis possible to switch back to a locking rate control when the off-roadcounter furnishes a corresponding result. To be able to quickly switchback into a traction control mode after its completion, it is advisableto shorten the monitoring time after completion of the previous tractioncontrol intervention.

Due to their principle, traction control interventions can have effectson the measured wheel slip λ and, consequently, on the vehicle referencespeed ν_(ref) determined therefrom. This is because the circumferentialspeed ν_(wheel) of an overbraked driving wheel in a traction controlmode differs significantly from the circumferential speed of a wheelthat rolls synchronously with the vehicle speed. A too low amount ofvehicle reference speed ν_(ref) can be determined as a result. Morespecifically, a driving wheel that is overbraked due to a tractioncontrol intervention will reduce the vehicle reference speed, what maycause control errors. It is therefore arranged for that the rotationalsignals of traction-controlled driving wheels are not taken into accountfor determining the vehicle reference speed, and that on detection of atraction control intervention on all wheels the driving wheel with thelowest locking torque (brake torque) due to traction control is takeninto account for determining the vehicle reference speed ν_(ref). Toapproach the vehicle reference speed to the actual conditions, anegative pressure gradient can be increased based on the assumption thatwith four slipping wheels the previously determined vehicle referencespeed was too high by a defined degree.

1. Method for traction control of slipping wheels of at least onedriving axle, the wheels of the at least one driving axle are driven byway of an open differential gear and can be interlocked by brakingintervention to a predeterminable degree for effecting a differentiallock function, the method comprising the steps of: sensing a wheel slipof the wheels; comparing a sensed wheel slip variation withpredetermined wheel slip patterns for detecting and monitoring a currentdriving condition; performing a braking intervention effecting thedifferential lock function when the sensed wheel slip variation iscoincident with at least one predetermined wheel slip pattern; whereinthe defined braking intervention includes an adaptive pressure reductiongradient or an adaptive pressure increase gradient is calculated on eachindividual wheel in dependence on the detected wheel slip pattern, andin that a pressure reduction or a pressure increase is realized byadapting pulse-pause valve actuation sequences; and wherein a pausebetween two pressure reduction pulses is determined as a function of avehicle reference speed, the wheel slip and a constant base quantityaccording to the relationΔT=T _(B) −k ₁ *ν _(ref) +k ₂*λ_(fil) where T_(B): base quantity k₁, k₂amplification factors ν_(ref): vehicle reference speed λ_(fil): wheelslip (filtered).
 2. Method as claimed in claim 1, wherein wheel slip ofthe wheels is monitored for determining the pause as a subtractivecomponent, and in that a progression or degression of a counter isexecuted in dependence on the wheel slip of the wheels according to theconditionk₃*Stab_(counter) where k₃ amplification factor Stab_(counter) counterfor monitoring the wheel slip.
 3. Method as claimed in claim 1, whereina component describing a torque difference between a current axle torqueand a filtered axle torque is subtractively taken into account accordingto the conditionk₄*ΔActTq where ΔActTq=ActTq_(fil)−ActTq and k₄: amplification factorActTq_(fil): axle torque (filtered) ActTq: axle torque (current). 4.Method as claimed in claim 1, wherein an engine speed and an enginespeed variation are subtractively taken into account according to theconditionk₅[k_(p)*(N_(nom)−N)+k_(d)*ΔN] where k₅, k_(p), k_(d): amplificationfactors N_(nom): nominal engine speed N: actual engine speed ΔN: enginespeed variation.
 5. Method as claimed in claim 1, further comprisingproviding an electronic controller based on a controller model withlinear components (P-components) and integral components (I-components).6. Method as claimed in claim 4, wherein a amplification factors (k₁₋₅,k_(p), k_(d)) are determined in dependence on the detected vehicledriving condition.
 7. Method as claimed in claim 1, wherein a controlthreshold is lowered after detection of a driving condition withtraction slip.
 8. Method as claimed in claim 1, wherein the differentiallock function is rendered plausible based on the vehicle reference speedand based on a cornering detection, and in that the differential lockfunction is deactivated in the event of unplausibility.
 9. Method asclaimed in claim 1, wherein the differential lock function isdeactivated when a driving condition is detected where all wheelsexhibit a low coefficient of friction or when cornering is considered toprevail.
 10. Method as claimed in claim 9, wherein a traction controlstrategy is provided on the basis of wheel-individual slip values inaddition to the traction control strategy on the basis of identifieddriving conditions, and in that the control takes place on the basis ofwheel-individual slip values when the differential lock function isdeactivated.
 11. Method as claimed in claim 1, wherein rotational dataof traction-controlled driving wheels is not taken into account fordetermining the vehicle reference speed, and in that when a tractioncontrol intervention is detected at all wheels, the driving wheel withthe lowest braking torque due to traction control is taken into accountfor determining the vehicle reference speed.
 12. Method as claimed inclaim 5, wherein the controller model further includes differentialcomponents (D-component, delay elements).
 13. Method as claimed in claim10, wherein control takes place on the basis of wheel-individual slipvalues when a driving condition is detected where all wheels adopt a lowcoefficient of friction.